1. Field of the Invention
This invention relates generally to powder metallurgy, and more particularly to iron-based powder metal articles for wear resistant applications, such as automotive valve guides.
2. Description of the Prior Art
Powder metal valve guides and other high temperature wear resistant articles are often formed from iron-based powder metal mixtures. Typically, the articles are formed by admixing various powder additives with an elemental iron powder, and then sintering the mixture at temperatures greater than 1000° C.
Lubricity of the powder metal article is often enhanced by admixing solid lubricants, such as molybdenum disulphide, with the elemental iron powder. Although admixed molybdenum disulphide is an excellent solid lubricant, it tends to undergo undesirable growth during the sintering process when present in amounts large enough to provide sufficient lubricity. The distortion associated with the molybdenum disulphide is detrimental to the manufacture of low cost, high precision, net shape articles, such as valve guides and valve seat inserts. Thus, high levels of molybdenum disulphide are typically avoided in powder metal applications.
Free graphite is another solid lubricant used in powder metal mixtures. Fine graphite particles, such as particles having a U.S. standard sieve designation of about 200 mesh or finer, are preferred over coarse graphite particles because they are easier to process and provide superior mechanical properties in the sintered article. However, the fine graphite particles will readily diffuse into elemental iron powders during sintering, and are thus unavailable to function as solid lubricant in the sintered article. For example, if a powder mixture including 1.0 wt % admixed fine graphite powder is sintered at a temperature above 1000° C., nearly all of the admixed graphite will readily diffuse into the elemental iron matrix during sintering and no significant levels of free graphite will remain in the final sintered article. In order to retain a useful level of free graphite in the final sintered article, it is necessary use admixed graphite having a particle size coarser than 200 mesh, so that the particle size limits diffusion of the admixed graphite into the elemental iron powder during sintering. However, the admixed graphite having a particle size coarser than 200 mesh often leads to processing difficulties and less desirable mechanical properties of the sintered article.
U.S. Pat. No. 5,507,257 discloses an iron-based powder metal mixture for valve guide applications including an elemental iron powder matrix, admixed coarse graphite (200 to 30 mesh), admixed fine graphite (finer than 200 mesh), and admixed ferro-phosphorous or admixed copper-phosphorous powder. As alluded to above, the admixed fine graphite is more reactive than the admixed coarse graphite and readily diffuses into the iron powder matrix during sintering. The admixed coarse graphite is less reactive due to the larger particle size and is specifically incorporated so that a significant level of free graphite is retained in the sintered article. However, as stated above, the admixed coarse graphite is prone to processing difficulties, such as undesirable powder segregation.
The sintered article of the '257 patent includes carbides when the mixture includes admixed molybdenum powder, hard Fe—C—P dispersions in the iron matrix, and free graphite due to the admixed coarse graphite. The admixed phosphorous powders promote sintering through formation of a transient liquid phase and have a stabilizing effect on the alpha-iron phase during sintering. The low carbon solubility in the alpha-iron phase promotes the beneficial presence of the free graphite in the sintered article. However, the admixed phosphorous is detrimental in that the partial liquid phase sintering can cause dimensional change upon solidification to such a degree that the tolerances of the sintered articles for net-shape applications may be adversely affected. Hard phosphorous compounds and cementite form at the grain boundaries as a result of the partial liquid phase sintering. The hard phosphorous compounds and cementite have a detrimental effect on the machinability and net-shape stabilization of the powder metal articles. Thus, the addition of phosphorous in iron-based powder metal applications is typically undesirable.
U.S. Pat. No. 6,632,263 also discloses an iron-based powder metal mixture for valve guide applications. The mixture includes an elemental iron powder matrix, admixed coarse graphite (325 to 100 mesh), admixed fine graphite (finer than 325 mesh), admixed molybdenum disulfide, and admixed copper. Like the mixture of the '257 patent, the admixed fine graphite of the '263 patent is more reactive and readily diffuses into the iron powder matrix during sintering, while the admixed coarse graphite is specifically incorporated to retain a significant level of free graphite in the final sintered article. Again, the admixed coarse graphite is prone to undesirable powder segregation during processing, and the coarse graphite particles may not retain desirable mechanical properties at high temperatures.